1. Field of the Invention
The invention relates to a dispersion compensating fiber module which is used for compensating the accumulated chromatic dispersion of a non-zero dispersion shifted optical fiber (NZ-DSF) which has chromatic dispersion of several ps/nm/km in the C-band (wavelength between 1.525 μm and 1.565 μm) and in the L-band (wavelength between 1.565 μm and 1.625 μm). More particularly, the invention relates to a dispersion compensating fiber module which is capable of suppressing residual dispersion to a low level, and to an optical fiber transmission line which is fabricated by connecting such a module to a transmission optical fiber.
2. Description of Related Art
In order to increase the transmission capacity of wavelength division multiplexing (hereinafter referred to as “WDM”), it is effective to increase the transmission rate for each channel by broadening the operating wavelength range. The relationship between the transmission rate and the residual dispersion is shown in FIG. 1. In order to increase the transmission rate, it is necessary to reduce the accumulated dispersion over the transmission line. In contrast to the case with 2.5 Gbit/sec transmission in which the allowable residual dispersion is about 16,000 ps/nm, with 10 Gbit/sec transmission it is about 1,000 ps/nm, and with 40 Gbit/sec transmission it is about 65 ps/nm. Thus, as the transmission rate is increased, the allowable residual dispersion become smaller. Due to this, when attempts are made to increase the transmission distance and to increase the transmission rate, dispersion compensation for each span becomes indispensable. Since such dispersion compensation is required over the entire operating wavelength range, it also becomes necessary to compensate the accumulated dispersion slope of the transmission line at the same time.
Numerous studies have already been reported regarding slope compensating and dispersion compensating fibers (SC-DCFs) for standard single-mode optical fibers (S-SMFs) (for example, refer to Japanese Unexamined Patent Application, First Publication No. H06-11620 and Japanese Unexamined Patent Application, First Publication No. 2002-221632).
Furthermore, similarly, SC-DCFs for NZ-DSFs have been reported which almost entirely compensate accumulated dispersion in a wide band (for example, refer to “High performance wide-band dispersion compensating fiber module for non-zero dispersion shifted optical fiber” by Kazuhiko Aikawa et al., Technology Report of the Institute for Electronics, Information and Communication Engineers, OCS 2002-7, April 2002, pp. 35–40).
FIG. 2 is a schematic diagram illustrating the basic concept of compensation for chromatic dispersion upon an optical fiber transmission line. A transmission optical fiber typically has a chromatic dispersion and a dispersion slope in both positive values. Therefore, it is possible to compensate the dispersion in a wide wavelength range by connecting an SC-DCF which has a negative chromatic dispersion and a negative dispersion slope having an appropriate length (i.e., a length in which the dispersion can be cancelled) with the ratio of the chromatic dispersion and the dispersion slope being appropriately adjusted. With actual transmission optical fibers and SC-DCFs, wavelength dependence in the dispersion slope is observed. In other words, the dispersion characteristic is a curved line, rather than such a linear line. Although the wavelength dependence of the dispersion slope of the transmission optical fiber is small as compared to that of the SC-DCF, the wavelength dependence of the SC-DCF is still comparatively large. In particular, in an SC-DCF, the greater the relative dispersion slope (RDS) with respect to the chromatic dispersion, the greater the wavelength dependence of the dispersion slope becomes.
FIGS. 3 and 4 are graphs showing examples of the residual dispersion characteristics when lengths of 80 km of various types of NZ-DSF, which are described in the above-described paper by Aikawa, have been dispersion compensated. In FIG. 3, the residual dispersion characteristic after a low dispersion slope type NZ-DSF has been compensated with an SC-DCF module is shown. In both of the C-band (wavelengths between 1.525 μm and 1.565 μm) and the L-band (wavelengths between 1.565 μm and 1.625 μm), the residual dispersion is less than or equal to ±5 ps/nm, and thus, it is possible to compensate the residual dispersion so that it is reduced over the entire wavelength range. This is because the curvature of the dispersion curve is small since the RDS of the SC-DCF is not particularly large. However, with the residual dispersion characteristic when a large effective area NZ-DSF shown in FIG. 4 has been compensated with an SC-DCF module, a relatively large residual dispersion of ±20 ps/nm remains in the C-band, and ±15 ps/nm in the L-band. This is because the curvature of the dispersion curve is large since the RDS of the SC-DCF is large. These values are both for a transmission optical fiber having a length of 80 km, when the residual dispersion are converted into lengths per km, they become less than or equal to ±0.25 ps/nm/km (the maximum residual dispersion difference is 0.5 ps/nm/km), and less than or equal to ±0.19 ps/nm/km (the maximum residual dispersion difference is 0.38 ps/nm/km). Since, in a long haul transmission, such residual dispersion accumulates in the similar manner, this entails a deterioration of transmission quality when dispersion compensation at each wavelength is not performed.
An SC-DCF has a refractive index profile as, for example, shown in FIG. 5. By adjusting the delta (Δ) and the ratio of the radii of the respective layers of this refractive index profile, it becomes possible to adjust the various types of optical property, including the RDS. In a range where the RDS is small, the design for this adjustment of the RDS and fabrication of such fibers can be achieved comparatively easily. However, when, for an SC-DCF having the RDS exceeds 0.01 nm−1, an attempt is made to maintain the bending loss and the cutoff wavelength and the like while making the absolute value of the chromatic dispersion great, the design and the fabrication become difficult, and furthermore the dispersion curve easily becomes severely curved.